1. Field of the Invention
The present invention concerns a cable for transmitting data comprising a conductive shield for protecting one or more conductors against external electromagnetic interference.
2. Description of the Prior Art
Data is usually transmitted by means of insulated electrical conductors surrounded by one or more metallic shields, the shield or shields being enclosed within a jacket.
The shield isolates the conductors from external electromagnetic interference. The best protection is obtained when the conductive shield is continuous, without openings in it. However, the usual manufacturing techniques impose the use of one or more tapes to form the shield which necessarily leads to openings in the latter which limit the efficacy of the shield at the highest frequencies.
The best shields provide effective protection up to frequencies in the order of 30 MHz to 40 MHz. Until now the best results have been obtained with a metal tape disposed lengthwise with the longitudinal edges overlapping. A lengthwise tape gives better results than a helically wound tape because the opening extends a shorter distance.
Increasing data bit rates in cables are leading to an increase in the limiting frequency below which cables are protected from the external environment.
U.S. Pat. No. 4,510,346 proposes a cable including at least one electrical conductor surrounded by a shield to protect against high-frequency electromagnetic interference. The shield includes an inner tape disposed lengthwise and an outer tape disposed lengthwise, each tape having a conductive layer covered by an insulative layer. The conductive layer of the inner tape faces outwards and the conductive layer of the outer tape faces inwards so that the conductive layers are pressed together. At least one of the two tapes has overlapping longitudinal edges.
Although the shield of a cable of the above kind provides effective protection against electromagnetic interference up to very high frequencies (at least 100 MHz), it nevertheless gives rise to a major problem, namely that of ease of stripping. The outer tape, disposed lengthwise, with its conductive layer on the inside against the conductive layer of the inner tape prevents easy access to the conductive part of the shield and therefore makes it difficult to connect the cable.
An aim of the present invention is to solve this problem by proposing a cable that is effective at very high frequencies, typically above 100 MHz, and easier to strip than prior art cables effective at such frequencies.